The Collaborative International Dictionary of English v.0.48:

Superimpose \Su`per*im*pose"\, v. t. [imp. & p. p.
   Superimposed; p. pr. & vb. n. Superimposing.]
   To lay or impose on something else; as, a stratum of earth
   superimposed on another stratum. -- Su`per*im`po*si"tion,
   n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

superimpose
    v 1: place on top of; "can you superimpose the two images?"
         [syn: superimpose, superpose, lay over]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "superimpose":
   apply to, blanket, block, canopy, cloak, clothe, cloud, cope,
   cover, cover up, cowl, curtain, eclipse, film, hood, lay on,
   lay over, mantle, mask, muffle, obduce, obscure, occult, overlay,
   overspread, put on, screen, scum, shield, spread over, superpose,
   veil